1. Field of the Invention
The present invention relates generally to methods and apparatus using ultrasonics in the field of medical technology and, more particularly, to an applicator for performing presurgical and surgical procedures using high-intensity focused ultrasound.
2. Description of Related Art
Therapeutic ultrasound refers to the use of high intensity ultrasonic waves to induce changes in living tissue state through both thermal effects--referred to in the art as induced hyperthermia--and mechanical effects--induced cavitation. High frequency ultrasound has been employed in both hyperthermic and cavitational medical applications, whereas low frequency ultrasound has been used principally for its cavitation effect. Diagnostic medical ultrasonic imaging is well known, for example, in the common use of sonograms for fetal examination. Various aspects of diagnostic and therapeutic ultrasound methodologies and apparatus are discussed in depth in an article by G. ter Haar, Ultrasound Focal Beam Surgery, Ultrasound in Med. & Biol., Vol. 21, No. 9, pp. 1089-1100, 1995, and the IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, November 1996, Vol. 43, No. 6 (ISSN 0885-3010), incorporated herein by reference. Particular methods and apparatus for medical applications of high intensity focused ultrasound, for example, for hemostasis and tissue necrotization, are the subject of pending U.S. patent application Ser. No. 08/961,972 (assigned to the common assignee of the present invention and incorporated herein by reference).
In high-intensity focused ultrasound (HIFU) hyperthermia treatments, intensity of ultrasonic waves generated by a highly focused transducer increases from the source to the region of focus, or focal region, where it can cause a high temperature effect, e.g. to 98.degree. Centigrade. The absorption of the ultrasonic energy at the focus induces a sudden temperature rise of targeted tissue--as high as one to two hundred degrees Kelvin/second--which causes the irreversible ablation of the target volume of cells. Thus, for example, HIFU hyperthermia treatments can cause necrotization of or around an internal lesion without damage to the intermediate tissues. The focal region dimensions are referred to as the depth of field, and the distance from the transducer to the center point of the focal region is referred to as the depth of focus. In the main, ultrasound is a promising non-invasive surgical technique because the ultrasonic waves provide a non-effective penetration of intervening tissues, yet with sufficiently low attenuation to deliver energy to a small focal target volume. Currently there is no other known modality that offers noninvasive, deep, localized focusing of non-ionizing radiation for therapeutic purposes. Thus, ultrasonic treatment has a great advantage over microwave and radioactive therapeutic treatment techniques.
Blood loss due to internal or external bleeding in trauma patients and hemorrhage in surgery is a major form of casualty.
Hemostasis is currently performed using intense heat, electrocautery, lasers, embolization, or application of extreme cold. HIFU offers an alternative as the sonic energy can be focused to a distant point within the body without damage to intervening tissue, allowing non-invasive hemostasis.
Various embodiments of ultrasonic applicators or probes generally include a manipulable transducer, having a power supply and electrical matching circuitry for driving the transducer, and a coupling device for guiding the ultrasonic energy from the face of the transducer to the site of the tissue to be treated. Coupling devices consist generally of a hollow members filled with water. Water provides excellent coupling of acoustic energy into tissue because of the similarity in their acoustic impedances; both media have a characteristic impedance of approximately 1.5 megarayls. However, it has been found that there are disadvantages in the use of the water-filled coupling for medical procedures. At the high intensities at which the device is operated, the water in the coupling device is prone to cavitation; the bubbles produced are disruptive to the ultrasound energy. Thus, degassed water must be used to reduce the chance for cavitation. Furthermore, a water-filled device in a surgical environment is very difficult to sterilize and it must be refilled with sanitary, degassed water each time it is used. If the coupling device ruptures, water leaks out and blood from the patient leaks in, further complicating surgical conditions. Extra tubes and equipment are required to pump and circulate the water within the cone, making for a complicated and cumbersome apparatus difficult to optimize for emergency rescue situations.
In the use of HIFU, another problem is that driving transducers at high voltage generates heat. The frequencies required lead to the need for thin, fragile transducer elements. Thus, a HIFU medical instrument has inherent design problems which can make an HIFU instrument hard to use manually and where overheating can cause transducer failure.
Thus, there is a need for improved ultrasound-to-tissue coupling devices. A simple, effective coupling device to to replace the water-filled coupling devices must be easily sterilized and prepared for use or quickly refittable with a prepackaged, sterile replacement. A solid-state coupling device must additionally resolve the existence of inherent shear modes which complicate and adversely affect the transmission of longitudinal mode ultrasonic energy, the more effective form for most therapeutic type medical procedures. Furthermore, there is a need for improving ultrasonic applicators amenable for use in catheteric medical procedures.